topic 7/17 - equilibrium Flashcards
describe when a state of equilibrium is reached in a closed system
when the forward and reverse reactions are occurring at the same rate and the concentrations of products and reactants remain constant
what is a closed system?
a sealed container
define homogeneous equilibrium
the equilibrium is for materials all in the same state/phase (ie gases)
define Le Chatelier’s Principle
the position of the equilibrium of a system changes to minimise the effect of any imposed change in conditions
describe the effect of a temperature increase on equilibria
if exothermic:
- the equilibrium will shift in the endothermic direction (to the left)
- yield of products decreases
if endothermic:
- the equilibrium will shift in the endothermic direction (to the right)
- yield of products increases
describe the effect of a pressure increase on equilibria
the equilibrium will shift to the side with the least amount of moles to oppose the change in pressure.
describe the effect of an concentration increase on equilibria
increase concentration of reactants - the equilibrium will shift towards the products (to the right) to oppose the change
increase concentration of products - the equilibrium will shift towards the reactants (to the left) to oppose the change
give the equation for the synthesis of ammonia
N2 (g) + 3H2 (g) <-> 2NH3 (g)
what is the compromise that needs to be achieved in the ammonia production industry?
conditions are often a compromise between best equilibrium yield and fastest rate to give a cheap product
describe the effect of a catalyst on equilibria
a catalyst increases the rate of reaction but has no effect on the equilibrium position as it increases the rate of the forward and reverse reactions equally. However, equilibrium is reached more quickly.
define the law of chemical equilibrium
at a given temperature the ratio of the concentration of products (raised to the power of their molar coefficients) to the concentration of reactants (raised to the power of their molar coefficients) is a constant.
give the equation for Kc:
mA + nB <-> pC + qD
Kc = [C]^p[D]^q/[A]^m[B]^n
significance of a small value of Kc (smaller than 1)
- higher concentration of reactant than product present at equilibrium
- equilibrium lies to the left hand side
significance of a large value of Kc (greater than 1)
- higher concentration of product than reactant present at equilibrium
- equilibrium lies to the right hand side
define heterogeneous equilibria
the substances involved are not all in the same state
why are any solid chemicals not included in the Kc expression?
Kc refers to concentrations in mol dm3 and the concentration of a solid does not change
how do you find the units of Kc?
you derive them
effect on Kc of:
- reversing the reaction
- halving the coefficients
- doubling the coefficients
- summing equations
- 1/Kc
- square root of Kc
- square of Kc
- Kc1 x kc2
effect of increasing temperature on the value of Kc for an exothermic reaction
Increasing the temperature drives the reaction in the reverse direction, increasing the reactant concentration and reducing the product concentration.
The vaue of Kc decreases correspondingly.
effect of decreasing temperature on the value of Kc for an exothermic reaction
Decreasing the temperature drives the reaction in the forward direction, decreasing the reactant concentration and increasing the product concentration.
The value of Kc increases correspondingly.
effect of increasing temperature on the value of Kc for an endothermic reaction
Increasing the temperature drives the reaction in the forward direction, increasing the product concentration and decreasing the reactant concentration.
The value of Kc increases correspondingly
effect of decreasing temperature on the value of Kc for an endothermic reaction
Driving the temperature drives the reaction in the reverse direction, decreasing the product concentration and increasing the reactant concentration
the value of Kc decreases correspondingly
describe the effect of a change in concentration on the value of Kc
when concentrations are changed the equilibrium shifts position so as to return to the original Kc value.
the shift in position is directed by the value of Kc being restored.
effect of pressure on Kc value
no change
effect of catalyst on Kc value
no change
if value of Kc<1,
equilibrium - almost entirely reactants
almost no reaction
if value of Kc>1,
equilibrium - almost entirely products
proceeds virtually to completion
if value of Kc=1
equilibrium - roughly in the centre
both reactants and products present
Define the reaction quotient, Q
measures the relative amount of products and reactants present during a reaction at a particular point in time
Q=
the equilibrium expression with non equilibrium concentrations:
aA + bB <-> cC + dD
Qc = [C]^c[D]^d/[A]^a[B]^b
if Q>Kc
the concentration of products is greater than at equilibrium and the reverse reaction is favoured until equilibrium is reached
if Q<Kc
the concentration of reactants is greater than at equilibrium and the forward reaction is favoured until equilibrium is reached
if Q = Kc
the system is at equilibrium and the forward and reverse reactions occur at equal rates
describe the relationship between Kc and Gibbs free energy change
at a given temperature, a negative ΔG value for a reaction indicates that the reaction is spontaneous and the equilibrium concentrations of the products are larger than the equilibrium concentrations of the reactants. Kc>1. The more negative the value of ΔG, the more the forward reaction is favoured and the larger the value of K.
K = 1
at equilibrium, neither reactants nor products are favoured. ΔG = 0
K>1
products favoured, ΔG <0 (negative value)
K<1
reactants favoured, ΔG>0 (positive value)
give two quantitative relationships between standard Gibbs free energy change, temperature, and the equilibrium constant
ΔG = -RTlnK
lnK = -ΔG/RT
